systemctl: append default suffix only if none present

systemctl: append default suffix only if none present

Simplify unit_name_mangle() and unit_name_mangle_with_suffix() to
always behave the same, and only append a suffix if there is no
type suffix. If a user says 'isolate blah.device' it is better to
return an error that the type cannot be isolated, than to try to
isolate blah.device.target.